Lord Denning said about Slynn in his capacity as such: "He was outstanding. The best I have ever known. He will go far."[1] His successful application to take silk in 1974 coincided with his becoming the first Leading Counsel to the Treasury.
He was appointed a Lord of Appeal in Ordinary on 11 March 1992, becoming a life peer as Baron Slynn of Hadley, of Eggington in the County of Bedfordshire,[3] and being sworn of the Privy Council. He was a dissenter in the case R v. Brown, which upheld the legality of the criminal convictions resulting from Operation Spanner.[4] As a member of the House of Lords, he served as Chairman of the House of Lords Select Sub-Committee on European Law and Institutions (1992–95), and as a member of the House of Lords Select Committee on Public Service (1996–98) and the Joint Parliamentary Committee on Corruption Bill (2003). He retired as a Law Lord in 2002.
He was appointed President of the Court of Appeal of the Solomon Islands in 2001[5] and was life President of the Lord Slynn of Hadley European Law Foundation and President of the Civil Mediation Council. From 1992-1996 he was President of The Academy of Experts.
He was a Trustee of The Loomba Trust, which cares for widows around the world, and Patron of the Bharatiya Vidya Bhavan Institute for Indian art and culture.